From what I read, excel is roughly 2.5% polycyclogluteracetal by weight.
You're actually making something closer to API's CO2 Booster
which is a 1.6% solution of gluteraldehyde in water. You should be able to make a 1/15th dilution of your 25% solution (1 part of your solution to 14 parts water) and it ought to work just fine. Dosing according to their instructions is 1 mL per 40 liters daily. I've been using this product in conjunction with DIY CO2 and have seen a significant difference in the growth of my plants and things like pearling in the afternoon/evening. It hasn't bothered any of my shrimp.
Take care with the 25% solution. The 1.6% solution is nothing to laugh about, but it takes less than a mouthful of the stronger stuff to kill a child. (LD50 for gluteraldehyde is only 134 mg/kg, so roughly 10mL of the 25% solution would be enough to kill 50% of 20kg children who swallowed it.) It's also fairly nasty stuff on your skin and inhaled. Like many things, it's perfectly safe if used and stored properly, but it's fairly awful if not.
Isn't glutaraldehyde the active ingredient in Excel. Don't people dose it in their planted and shrimp tanks?
More or less, excel contains polycyclogluteracetal, basically polymerized gluteraldehyde. I imagine it's a stability issue, both in the tank and in the bottle. In any case, if you spray it onto the plants it can cause problems, and it's reasonably toxic stuff.